1. Field of the Invention
The present invention relates to polymer electrolytes for use in polymer electrolyte fuel cells, and also to membrane-electrode assemblies and fuel cells.
2. Description of the Related Art
In recent years, fuel cells have been attracting attention as an effective solution to environmental or energy problems. Fuel cells use an oxidant such as oxygen to oxidize the fuel such as hydrogen and convert the chemical energy produced by the oxidation into electric energy.
According to the electrolyte type, fuel cells are classified into an alkali type, a phosphoric acid type, a polymer electrolyte type, a molten carbonate type, a solid oxide type, and the like. Polymer electrolyte fuel cells (PEFC) are expected to be applicable to portable power sources, household power sources, and vehicle power sources, because they can work at low temperature, have high power density and be made small and light.
Electrolytes that have practical stability and are used for polymer electrolyte fuel cells (PEFC) include perfluoropolymer electrolytes typified by Nafion (a registered trademark of DuPont; hereinafter the same shall apply) and various hydrocarbon-based electrolytes. However, these electrolytes have the problem of high cost, although the electrolytes exhibit high proton conductivity.
In order to solve the problem, inexpensive hydrocarbon-based electrolytes have been developed. For example, Japanese Patent Application Laid-Open (JP-A) Nos. 11-116679 and 2007-329120 report some sulfonated engineering plastics. However, these electrolytes have reduced proton conductivity under low humidity conditions, although the electrolytes exhibit high proton conductivity under high humidity conditions. A method of increasing the concentration of the protic acid group has been attempted to produce high proton conductivity even under low humidity conditions. However, the increase in the concentration of the protic acid group has a problem in which the water resistance of the film is reduced so that the mechanical strength to withstand practical use cannot be achieved.
On the other hand, there is proposed a method of crosslinking an electrolyte to improve the durability, water resistance and mechanical strength of a hydrocarbon-based electrolyte membrane (JP-A No. 2007-70563). However, since the crosslinking reaction proceeds with the protic acid group bonded to the proton-conducting polymer, the hydrogen ion exchange capacity of the crosslinked electrolyte is decreased with increasing crosslink density. Therefore, this method has the problem of a reduction in proton conductivity. The crosslinked electrolyte proposed in JP-A No. 2004-10677 has a problem in which a certain crosslink group has to be introduced into the electrolyte, so that the crosslinked electrolyte is not easy to produce.
<Patent document 1>JP-A-H11-116679
<Patent document 2>JP-A-2007-329120
<Patent document 3>JP-A-2007-070563
<Patent document 4>JP-A-2004-010677